Internal combustion engines generate drive torque that is transferred to a drivetrain via a crankshaft. Some internal combustion engines include engine control systems that deactivate cylinders under low load situations. For example, an eight cylinder engine can be operated using four cylinders to improve fuel economy by reducing pumping losses. This process is generally referred to as displacement on demand or DOD. Operation using all of the engine cylinders is referred to as an activated mode. A deactivated mode refers to operation using less than all of the cylinders (i.e., one or more cylinders not active).
The engine is controlled using a torque control system that regulates engine operating parameters including, but not limited to, airflow into the engine, electronic spark timing, fueling and the activation or deactivation of the cylinders. The torque control system regulates these engine operating parameters based on the various engine operation inputs and a driver requested engine torque.
In existing torque control systems, various sub-systems regulate engine operation. For example, the throttle controls engine intake manifold pressure (i.e., air load) and airflow, the electronic spark control controls the spark timing and fuel injectors meter fueling. In combination, these sub-systems regulate the engine torque output. Assuming that fueling is fixed at a stoichiometric ratio for emissions control, fueling cannot be used to control torque in a normal operating mode. This leaves airflow, air load and spark timing as the sub-systems that can regulate engine output torque.
Some engines implement a spark knock limit (i.e., a maximum spark advance that keeps the engine from knocking) that is determined based on cylinder air load. In a deactivated mode, high cylinder air loads are achieved and the spark timing is greatly retarded to prevent knock. As the spark timing is retarded further from the maximum spark for best torque value (MBT), the engine's torque output decreases. As a result, the throttle is opened to increase the load in order to keep the engine torque output constant. However, as the throttle opens, air load is increased and the spark timing is further retarded. This cycle continues until the engine is fully un-throttled and must return to the activated mode (i.e., operating on all cylinders). At this point, any efficiency gains that result from operating in the deactivated mode are lost.